endocytosis
(rus. эндоцитоз)
— adsorption of substances by cells by invaginating a part of a cell membrane and pinching off to form membraneenclosed vesicle (endosome) with extracellular content in cytoplasm. The opposite process is called exocytosis.

Description

Cells use endocytosis to recycle surface receptors, to import selected extracellular macromolecules and to engulf bacteria, viruses and other pathogenic agents as part of the immune defence. Substances to be degraded go to lysosomes. Endocytosis pathways could be subdivided into several categories: phagocytosis (receptor-mediated capture of large particles, such as bacteria and dead cells), macropinocytosis (formation of large intracellular vesicles filled with extracellular material), caveolae pathway (capture of the substance by small vesicles 50-100 nm in size), receptor-mediated endocytosis (see Fig.).

Endocytosis is the central process in the delivery of nanodrugs and therapeutic genes to a cell. On the one hand, it can be used for the targeted delivery of nanoparticles to cells; on the other, it can mediate side effects. For example, by functionalising carbon nanotubes with phosphatidylserine (a lipid, prophagocytic signal for macrophages) the tubes can be targeted to macrophages using the phagocytic pathway, and by conjugating magnetic nanoparticles with tumour marker antibodies the particles can be targeted to cancer cells using the receptor- mediated endocytosis. Toxic effects of nanoparticles, such as semiconductor quantum dots used for in vivo diagnostics, correlate directly with the level of their endocytosis-mediated internalisation intocells.